1. Field of the Invention
This invention relates to a scanning optical apparatus, and particularly to a scanning optical apparatus suitable for use in an apparatus, such as a laser beam printer (LBP), having an electrophotographic process or a digital copying apparatus designed to deflect a light beam emitted from light source means, comprising a semiconductor laser, by a deflecting element, and optically scan a surface to be scanned through a scanning optical element (an imaging element) having the f.theta. characteristic to thereby record image information.
2. Related Background Art
In a scanning optical apparatus used in a laser beam printer, a digital copying apparatus or the like, a light beam optically modulated and emitted from light source means in conformity with an image signal is periodically deflected by a light deflector comprising, for example, a rotatable polygon mirror, and is converged into a spot-like shape on the surface of a photosensitive recording medium (a photosensitive drum) by a scanning optical element (an imaging element) having the f.theta. characteristic, and the surface is optically scanned to thereby effect image recording.
In recent years, it has become more desirable for scanning optical apparatus used in such a laser beam printer (LBP) or the like, to maintain their accuracy during fluctuations in the environment and be suitable for highly minute printing with the reduction in price and compactness of the LBP.
FIG. 1 of the accompanying drawings is a schematic view of the essential portions of a scanning optical apparatus of this kind according to the prior art.
In FIG. 1, a divergent light beam emitted from light source means 21 is substantially collimated by a collimator lens 22, and this light beam (the quantity of light) is limited by a stop 23 and enters a cylindrical lens 24 having predetermined refractive power only in a sub-scanning direction. In the main scanning cross-section of the parallel light beam which has entered the cylindrical lens 24, the light beam emerges therefrom intactly as a parallel light beam. Also, in the sub-scanning cross-section, the light beam converges and is formed as a substantially linear image on the deflecting surface (reflecting surface) 25a of a light deflector 25 comprising a rotatable polygon mirror. Here, the term main scanning cross-section refers to a light beam cross-section formed with time by a light beam deflected and reflected by the deflecting surface of the light deflector. Also, the term sub-scanning cross-section refers to a cross-section containing the optical axis of an f.theta. lens and which is orthogonal to the main scanning cross-section.
The light beam deflected by the deflecting surface 25a of the light deflector 25 is directed to a photosensitive drum surface 28 as a surface to be scanned through a scanning optical element (f.theta. lens) 26 having the f.theta. characteristic, and the light deflector is rotated in the direction of arrow A, whereby the photosensitive drum surface 28 is optically scanned in the direction of arrow B. Image recording is thereby effected on the photosensitive drum surface 28, which is a recording medium.
In recent years, as regards the scanning optical element (f.theta. lens) in the scanning optical apparatus of this kind, one using a plastic lens formed of a plastic material has become mainstream from the requirements for a low price and compactness. However, the refractive index of a plastic lens varies with fluctuation (temperature fluctuation) of in its use environment and therefore, in a scanning optical apparatus using an f.theta. lens comprising a plastic lens, changes in the magnification in the main scanning direction and changes in focus due to the fluctuation of the environment are caused.
FIG. 2 of the accompanying drawings is a cross-sectional view (main scanning cross-sectional view) of the essential portions of a comparative example of the scanning optical apparatus in the main scanning direction thereof for illustrating such a problem, and Table 1 below shows the optical arrangement and the aspherical coefficient of the f.theta. lens in the comparative example. FIG. 3 of the accompanying drawings is an illustration showing the curvature of the image field, the aberration of distortion and image height deviation in the main scanning direction of this scanning optical apparatus, and the solid lines indicate the characteristics at room temperature (25.degree. C.), and the dotted lines indicate the characteristics when a temperature rise of 50.degree. C. arose. In FIG. 2, the same elements as those shown in FIG. 1 are given the same reference characters.
TABLE 1 __________________________________________________________________________ Design Example of Scanning Optical Apparatus __________________________________________________________________________ surface shape of f.theta. lens first surface second surface wavelength used (nm) 780 R 5.35941e + 01 2.04585e + 02 refraction n 1.525 Ks -1.85041e + 01 -3.19655e + 02 index of f.theta. lens incident angle in polygon .theta.i -90.0 B4s -4.01467e + 06 -5.67674e - 06 max. emergence angle in polygon .theta.max 45.0 B6s 1.97617e - 10 1.13298e - 09 distance between polygon e 21.3 B8s 3.18251e - 13 -1.08244e - 12 and f.theta. lens center thickness of f.theta. lens d 8.0 B10s -4.34340e - 17 3.75204e - 17 distance between f.theta. lens and Sk 128.2 Ke -1.85041e + 01 -3.19655e + 02 scanned surface f.theta. coefficient f 136.0 B4e -6.38051e - 06 -7.32456e - 06 polygon .phi.20, B6e -5.04862e - 10 4.21805e - 10 4 surfaces In the shape of f.theta. lens, B8e 2.89411e - 13 -1.75629e - 12 suffix s indicates laser side, B10e 1.05151e - 15 5.30015e - 17 and BOE Phase term suffix e indicates a side opposite to laser side. first surface second surface C2 -- -- C4 -- -- C6 -- -- C8 -- -- __________________________________________________________________________
As can be seen from FIG. 3, when an f.theta. lens comprising a plastic lens is used, the focus and magnification in the main scanning direction are changed greatly by a temperature rise, and particularly in a scanning optical apparatus for effecting highly minute printing, the changes in the focus and magnification due to this environmental fluctuation (temperature fluctuation) pose a problem.
Also, from U.S. Pat. No. 5,486,694, there is known a scanning optical apparatus in which a light beam emitted from light source means is deflected by a deflecting element, and the deflected light beam is imaged on a surface to be scanned through a scanning optical element having a refracting portion and a diffracting portion to thereby scan the surface to be scanned.
In the scanning optical element disclosed in this U.S. Pat. No. 5,486,694, however, no consideration is paid to the environmental fluctuations.